The present invention relates to an active matrix type of picture image display device and to a method of driving the same, in which the light emitting operation of an electro optical element, whose emission is caused by a drive current flowing through a light emitting thin film, such as an organic semiconductor film, is controlled by switching elements, such as thin film transistors (hereinbelow referred to as TFTs).
In response to the advent of a highly information oriented society, demands for devices, such as personal computers, car navigation devices, portable information terminals, information communication products and combined devices thereof, have been increasing. For these products, thin display devices that are light in weight with low electric power consumption are desirable; therefore, liquid crystal display devices or display devices including self light-emitting type electro optical elements, such as electro-luminescence elements (hereinbelow referred to as ELs) and light emitting diode elements (hereinbelow referred to as LEDs) are now being used.
Since the latter display devices including the self light-emitting type electro optical elements show advantages such as high visibility, broad visual angle and high response characteristics, which are suitable for displaying motion pictures, such display devices are recognized as being particularly suitable for use in the future, because video image display will become of major importance. In particular, a rapid improvement in light emitting efficiency in the case of organic EL elements and organic LED elements (hereinbelow referred to inclusively as OLEDS) using an organic substance as a light emitting layer and development in network technology which permits video image signal communication in combination therewith have greatly increased the expectation for wide-spread use of OLED displays.
In order to enhance the power efficiency in OLED displays, an active matrix drive using thin film transistors is effective which will be explained later. Examples of technology in which an OLED display is constituted in an active matrix structure and is driven therewith are, for example, disclosed in JP-A-4-328791 (1992), JP-A-8-241048 (1996) and U.S. Pat. No. 5,550,066; and, further, a technique relating to a drive voltage of such OLED display is, for example, disclosed in PCT application laid-open No. WO98/36407.
In a typical pixel in an OLED display, the light emitting intensity of an OLED element is controlled by an active matrix element drive circuit, which is constituted by two TFTs (one is for a switching transistor and the other is for a driver transistor) and one capacitor. Pixels are arranged near respective cross points of m lines and n rows arranged in the form of a matrix, which is formed by n data lines to which picture image signals are fed and m scanning lines (or gate lines) to which scanning signals are fed.
In order to drive the pixels, through successive application of scanning signals (or gate voltages) onto the m gate lines, the respective switching transistors are turned on to complete scanning in a vertical direction once in one frame period Tf, and a turn-on voltage is again applied to the first gate line.
In the above pixel drive scheme, an interval when the turn-on voltage is applied to a single gate line is less than Tf/m. As a value for one frame period Tf about 1/60 sec is generally employed. At the time-on voltage is being applied on a certain when a turn gate line, all of the switching transistors connected to the certain gate line are placed in an on state, and in synchronism therewith picture image signals (data voltages) are applied at the same time to data lines of n rows, which is called a line-at-a-time scanning method and is commonly used in the field of active matrix type liquid crystal display devices.
The data voltage is stored in the capacitor during the time when the turn-on voltage is applied to the gate line and is kept there at substantially the same value during one frame period. The voltage value at the capacitor defines the gate voltage of the driver transistor, thus the value of current flowing through the driver transistor is regulated so as to cause a predetermined current to flow through the OLED element and to cause light emission. The response time of an OLED element in starting light emission after a voltage is applied is usually below 1 μs, therefore, the OLED element can be operated so as to follow a swift motion picture image (motion picture).
Now, with use of the active matrix drive, a high efficiency is realized, because the light emission is effected during the entire one frame period. When comparing this active matrix drive with a simple matrix drive in which an OLED element is driven by connecting the respective diode electrodes thereof to a vertical scanning line and a horizontal scanning line without providing the TFTs, the difference in efficiency is significant.
In a simple matrix drive, the current flows through the OLED element only during the period when the vertical scanning line is selected; therefore, in order to obtain a brightness equivalent to that provided by light emission in one frame period only with such a short period light emission, a light emission intensity multiplied by about the vertical scanning line number is required in comparison with that for the active matrix drive. For fulfilling such a requirement, the drive voltage and the drive current therefor have to be inevitably increased, which increases the loss power consumption, such as heating, and can not prevent a power efficiency reduction.
As will be apparent from the above, it is believed that the active matrix drive is superior to the simple matrix drive with regard to its lower power consumption. Because of the high speed response characteristics of the OLED elements, it has been believed that such OLED elements are suitable for motion picture displays. However, the active matrix drive for such OLED elements according to the conventional art employs the same drive method as used for liquid crystal displays (LCDs), namely a hold type display method, in which the respective pixels are displayed, in other words, the respective OLED elements are caused to light-emit over one frame period.
Hidekazu Ishiguro et al. “Consideration on Motion Picture Quality of the Hold Type Display with an Octuple-rate CRT” (Technical Report of The Institute of Electronics, Information and Communication Engineers, EID 96-4 (1996-06), pp 19-26) discloses that it is unavoidable in a LCD that edges of a moving body will be blurred during motion picture display due to the hold type display method.
Although the problem of blurred edge of motion picture images was pointed out with regard to LCDS, the cause of the blurred edges is stated as being due to use of the hold type display method. Therefore, the same problem of blurred edges of motion picture images likely arises with regard to active matrix driven OLED elements, when such OLED elements are operated with the hold type display method.
As has been mentioned above, in the conventional art the blurred edges phenomenon during motion picture display when electro optical elements, such as OLED elements, are driven by active matrix circuits was not taken into account, which causes deterioration of the picture quality thereof.